During World War I, all the Allies agreed on the reconstitution of Poland that United States President Woodrow Wilson proclaimed in Point 13 of his Fourteen Points. Shortly after the armistice with Germany in November 1918, Poland regained its independence as the Second Polish Republic (II Rzeczpospolita Polska). It reaffirmed its independence after a series of military conflicts, the most notable being the Polish–Soviet War (1919–1921) when Poland inflicted a crushing defeat on the Red Army.Grave of Polish fighter killed during the Warsaw Uprising. In the battle, which lasted 63 days, more than 200,000 people died.

The 1926 May Coup of Józef Piłsudski turned rule of the Second Polish Republic over to the Sanacja movement.